Use flag for RCLASS_IS_INITIALIZED

Previously we used a flag to set whether a module was uninitialized.
When checked whether a class was initialized, we first had to check that
it had a non-zero superclass, as well as that it wasn't BasicObject.

With the advent of namespaces, RCLASS_SUPER is now an expensive
operation, and though we could just check for the prime superclass, we
might as well take this opportunity to use a flag so that we can perform
the initialized check with as few instructions as possible.

It's possible in the future that we could prevent uninitialized classes
from being available to the user, but currently there are a few ways to
do that.

eval.c

@@ -422,7 +422,8 @@ rb_class_modify_check(VALUE klass)
         Check_Type(klass, T_CLASS);
     }
     if (RB_TYPE_P(klass, T_MODULE)) {
-        rb_module_set_initialized(klass);
+        // TODO: shouldn't this only happen in a few places?
+        rb_class_set_initialized(klass);
     }
     if (OBJ_FROZEN(klass)) {
         const char *desc;